The Nucleus Accumbens (NAc) is important for the learning and expression of reward-related behaviors, and strongly influenced by drugs of abuse. Medium spiny neurons (MSNs) are the principal cells of the NAc, and are commonly segregated into two distinct populations. MSNs expressing D1 dopamine receptors project directly to the midbrain (D1-MSNs), while MSNs expressing D2 dopamine receptors project indirectly via the ventral pallidum (D2-MSNs). D1-MSNs and D2-MSNs process long-range excitatory inputs from the prefrontal cortex, thalamus, ventral hippocampus and basolateral amygdala. Recent findings from our laboratory indicate that synaptic connectivity in the NAc is both cell-type and input-specific, with hippocampal inputs stronger at D1-MSNs, and other long-range inputs largely unbiased. Glutamatergic inputs ultimately synapse onto spines, the small membrane protrusions found throughout the dendrites of D1-MSNs and D2-MSNs. Each spine receives a single synaptic contact, and possesses the machinery for postsynaptic transmission, including both AMPA and NMDA receptors. The types and locations of targeted spines dictate postsynaptic responses, with inputs onto smaller or distal spines evoking weaker signals. Recent studies from our laboratory, using a novel combination of two-photon microscopy and optogenetics, reveal that subcellular connectivity is also cell-type and input-specific. Thus, hippocampal inputs selectively contact larger, proximal spines at D1-MSNs, whereas other long-range inputs show limited preference. Cocaine and other drugs of abuse are well known to have dramatic effects on both the morphological and physiological properties of MSNs in the NAc. Cocaine sensitization is a hyper-responsiveness to repeated cocaine exposure, and promotes the growth of new spines and formation of new synapses. These classical observations suggest a rearrangement of functional circuits, but the types of long-range excitatory inputs that are impacted remain unknown. New preliminary data from our lab indicates that repeated cocaine exposure triggers dramatic changes to cell-type and input-specific synaptic connectivity. Our proposed experiments will characterize how cocaine sensitization impacts the synaptic organization of the NAc at the levels of neurons, dendrites and spines. This work involves a powerful combination of whole-cell recordings, optogenetics, two-photon microscopy, two-photon uncaging, in vivo pharmacology and in vivo pharmacogenetics. Our long-term goal is to further our understanding of how cocaine and other drugs of abuse reorganize striatal circuits during the transition to drug addiction.